Microplastics (MPs), defined as plastics with diameters between 1 and 5000 µm, are problematic pollutants in the environment, but their removal is challenging because of their minute size. One promising approach for their removal is flotation because MPs are inherently hydrophobic. However, the very small particle size of MPs lowers the probability of MPs-bubble collision and attachment that in turn affects the efficiency of the process. To address this challenge, we propose the use of agglomeration-flotation, a technique using kerosene as a bridging liquid to enlarge the particle sizes of MPs and make them amenable to flotation. In this study, the effects of kerosene dosage on particle size enlargement and floatability of six types of MPs with 100–1000 µm size fractions were investigated. The results showed that MPs with lower density compared with water could easily float in water without bubble attachment and particle agglomeration required. So, the effects of agglomeration on removal were negligible. In contrast, agglomeration using kerosene enhanced the floatability of MPs with high-density plastics. Moreover, image analysis was used to determine the agglomerated MPs’ particle size. The results indicate that kerosene could agglomerate the MPs and enhanced the removal of MPs by agglomeration-flotation.

We developed a technique called the reverse hybrid jig, an advanced physical separation technique that combines the principles of jig and flotation to separate floating plastics. This technique is a promising green technology that is more economical and environmentally friendly compared with the conventional flotation. Although the applicability of this technique to separate PP/PE have been reported, the index to illustrate the possibility of separation for the reverse hybrid jig is still not available. In this study, a reverse apparent concentration criterion (CCRA) is proposed to estimate reverse hybrid jig separation efficiency. This modified concentration criterion can be calculated using the specific gravity (SG) of particle with attached bubbles called the apparent specific gravity (SGA). To determine the volume of attached bubbles on plastic surfaces under water pulsation, a laser-assisted apparatus was used under various conditions, including plastic type, air flow rate, dosage, and type of wetting agent. The results of attached bubble volume measurements were used to calculate the SGA and CCRA. The estimated values were then compared with the results of reverse hybrid jig separation. It was found that higher CCRA resulted in better separation efficiency. In addition, an empirical linear equation for estimating the reverse hybrid jig separation efficiency is proposed.

In this study, the microencapsulation-hybrid jig separation technique was developed to improve the separation efficiency of pyrite and coal in the particle size range of 1–4 mm where conventional jig separation becomes inefficient. A hybrid jig is a gravity concentrator combining the concepts of jig separation and flotation to stratify particles based on their apparent specific gravity. Meanwhile, microencapsulation—a technique that encapsulates target materials with a protective coating—was applied to render pyrite hydrophilic and improve its separation from hydrophobic coal. The results showed that the required time for separation in the hybrid jig (0.5 min) was shorter than in conventional jig (2 min). Moreover, the effects of particle size on separation efficiency were reduced when a hybrid jig is used. However, the separation efficiency of hybrid jig separation was lower than that of the conventional jig because attachment of bubbles occurred to both pyrite and coal, which are hydrophobic. Using the microencapsulation-hybrid jig separation technique, the separation of coal and pyrite was significantly improved (~100%) because of the formation of hydrophilic iron phosphate coatings on pyrite that limited bubble attachment. This means that microencapsulation-hybrid jig separation is a promising clean coal technology that not only enhances the separation efficiency of the hybrid jig but also passivates pyrite and limits AMD formation in the tailings/rejects.

Rare earth minerals (REMs) contain rare earth elements (REEs) that are important in modern technologies due to their unique magnetic, phosphorescent, and catalytic properties. However, REMs are not only non-renewable resources but also non-uniformly distributed on the Earth’s crust, so the processing of REE-bearing secondary resources via recycling is one potential route to ensure the long-term sustainability of REE supply. Flotation—a method that separates materials based on differences in their surface wettability—is a process applied for both mineral processing and recycling of REEs, especially when the particles are fine and/or a high-purity product is required. In this review, studies about rare earth flotation from 2012 to 2021 were systematically reviewed using the PRISMA guideline. It was found that most REM flotation research works focused on finding better collectors and depressants while, for recycling, studies on advanced flotation techniques like froth flotation, ion flotation, solvent sublation, electroflotation, and adsorbing colloid flotation with an emphasis on the recovery of dissolved REEs from aqueous solutions dominated.